The present disclosure relates to methods and systems for global positioning, and more specifically, to methods and systems for acquiring global positioning system signals from satellites to determine a location of a user receiver.
The Global Positioning System (GPS), a world-wide radio navigation system, provides an ability to obtain real time location and position information for mobile platforms and/or individuals. GPS includes a constellation of satellites, known as space vehicles, ground or base stations, and user receivers.
Using the locations of satellites as reference points, the position of the user receiver may be calculated accurately to within meters and sometimes even within centimeters. Each of the satellites, the ground stations, and the user receiver has preprogrammed timed signals that initiate at precise times. In order to lock on to the signals broadcasted by the satellites, the ground station and user receiver slew their respective internal generated signals relative to time as predicted by their respective internal clocks. When the signals are locked, each user receiver may calculate ranging measurements to each satellite called pseudo ranges, by determining the delays in the signal transmissions from the satellites. The pseudo range measurements include the actual ranges to satellites, in addition to an error associated with the receiver clock time offset relative to GPS time, plus other smaller errors. The ground stations included in the GPS provide ranging measurements that are used to generate predictions for the satellites, clocks, and orbits.
Signal acquisition is an important phase associated with the GPS receiver. Known GPS navigation systems are synchronized when an exact copy of the space vehicle's specific pseudo-random noise code of the received signal is available to demodulate data properly, a process characterized as coherent. However, if the received GPS signal is distorted or corrupted by factors such as, for example, noise, interfering signals and/or jamming, the signal is non-coherent. GPS receivers are needed that enable non-coherent detection and time recovery for distorted signals.